From US 2007/0053671 A1 a method is known to detect rain by focusing on and counting individual drops. Thereby rainfall is measured using an image acquisition device by first adjusting the image acquisition device's settings to enhance the visibility of rain, including adjusting at least one of the focal length, the F-number, and the exposure time. Second a plurality of images is acquired with the image acquisition device. Third a measure of the rain rate is computed based on counting the number and size of raindrops in each of the plurality of images. Disadvantageously this method needs to focus on the drops, which means that the background will probably be out of focus. This is a drawback because in order to combine such a method with a driving assistant system primarily interested in detecting e.g. the bounds of a roadway or lane, an extra image acquisition device like e.g. an extra video camera is required to bring the roadway into face.
From US 2007/0047809 A1 an environment recognition device is known, which acquires environment information and makes color-based decisions to determine what is seen. Disadvantageously thereby knowledge of the color of things is required. Particularly this is a drawback because color is not always available, e.g. either since a color camera is not being used, or since at night color information is significantly reduced.
From U.S. Pat. No. 6,831,261 a headlamp control system for a motor vehicle is known, which controls a headlamp of the motor vehicle in response to identification of at least one of a headlamp of another vehicle. The headlamp control system can also determine weather conditions. To determine weather conditions the fact is used that at fog or rain a bright light source of a head- or taillight of an oncoming or a preceding motor vehicle is surrounded by a transition region between the intensity of the light source and the black background. By placing appropriate limits on the size of the transition region, fog or light rain, or a mixture of both or other weather conditions can be detected. Furthermore fog or fine rain is detected by analyzing the effects of headlights of oncoming motor vehicles as reflected off of moisture particles in the air. Additionally spatial filtering is used to detect rain on the windshield. Thereby the effect is used that when a droplet of rain or a snowflake is on the windshield, a lack of continuous variation of differences between adjacent pixels or pixel groups of an imaging array sensor is created. This has the tendency to reduce the first derivative of the pixel, a condition which can be determined by processing. Disadvantageously also color information is required to distinguish between head- and taillights in the field of view. Another drawback of the headlamp control system is, that it looks for raindrops or snowflakes on the windshield and/or that it analyzes the effects of headlights of oncoming motor vehicles as reflected off of moisture particles in the air. Thus the headlamp control system is only capable to detect e.g. rain or snow during rain- or snowfall and thus during raindrops or snowflakes are on the windshield or in the air, but it is not capable to detect e.g. a wet or snow-covered road, like e.g. after rain- or snowfall.
From U.S. Pat. No. 6,313,454 a rain sensor system for a motor vehicle is known, looking for raindrops on a window of a motor vehicle, e.g. on the windshield. Thereby an imaging array sensor is directed toward a window of a motor vehicle. The rain sensor system counts the number of visible edges, using extra illumination if required at night. A drawback of this rain sensor system is that an extra imaging array sensor is required directed toward the window, which imaging array sensor cannot be used to detect e.g. road lane markings and road boundaries, obstacles, other road users and the like, or to survey and/or display the frontal and/or back area of the motor vehicle.